Ferrous base with nickel-iron coating



June 13, 196 7 MAYER ETAL 3,325,259

' FERROUS BASE! WITH NICKELIRON comma Filed May 13, 1964 4 Sheets-Sheet1 INVENTORS Edward H. Mayer Hi/fon N. Ra/m Jr. R/bhard M. Will/son June13, 1967 MAYER ET AL FERROUS BASE WITH NICKELIRON COATING 4 Sheets-Sheet2 Filed May 13, 1964 QQQ 33c v8 k mumkum S E EmQEm QQQ QN hm \MJWNNC 5 3INVENTORS Edward H. Mayer Hilton /V. Rah/2 Jr.

Richard/14. Wi/l/san June 13, 1967 MAYER ET AL 3,325,259

FERROUS BASE WITH NICKEL-IRON COATING Filed May 13, 1964 4 Sheets-Sheet5 Distance from surface in mills a a a g & Q Q

INVENTORS l w/1v Edward H. Mayer H/lfon IV. Rah/7 Jr. R/tharo M.W/l/ison June 13,1967 E. H. MAYER ETAL 3,325,259

FERROUS BASE WITH NICKEL-IRON COATING Filed May 13; 1964 INVENTORSEdward H. Mayer Hi/fon /V. Ra/m Jr. R/bhara' M. Will/S00 United StatesPatent 3,325,259 FERROUS BASE WITH NICKEL-IRON COATING Edward H. Mayer,Hilton N. Rahn, Jr., and Richard M.

Willison, Bethlehem, Pa., assignors, by mesne assignments, to BethlehemSteel Corporation, a corporation of Delaware Filed May 13, 1964, Ser.No. 367,182 6 Claims. (Cl. 29-196.1)

This invention relates to a ferrous-nickel alloy coating for .steelarticles, and to the formation of such coat- The principal object ofthis invention is to produce a corrosion resistant iron-nickel alloycoating on steel surfaces such as those of sheet and strip.

Another object is to produce a dense, impervious nickelcontainingcoating for steel articles, which coating is subsequently electrocoatedwith a decorative metal.

A further object is to form a nickel-containing coating by theapplication of nickel powder to the steel surface.

Generally, heretofore, when nickel-containing protective coatings havebeen used on the surfaces of steel sheets, strips, bars, etc., thecoating has taken the form of a substantially pure nickel layer on thesteel or iron substrate. These coatings have been formed principally byelectrodeposition, or by cladding with nickel strip.

We have found that by applying a uniform layer of finely divided nickelpowder to the surface of a steel article, such as strip or sheet, undercontrolled conditions, a ditfused iron-nickel alloy coating is producedwherein the alloying of iron and nickel is complete, i.e. all of thenickel powder alloys with diffused iron. Furthermore, by varying theoperating conditions, the iron-nickel ratio can be varied within certainlimits.

In addition to having excellent adherence and corrosion resistancecharacteristics, which characteristics render the alloy useful as aprotective coating, the alloy of our invention has a dense structure,which fact makes the alloy useful as an undercoat for certain porousdecor-ative metals, such as electrodeposited chromium.

In accordance with this invention, a steel sheet or strip is coated witha thin film of liquid, the film acting as a temporary bonding agent forsubsequently applied metal powder. A metal powder of nickel is nextapplied uniformly over the surface of the strip, the powder being heldin place by the previously applied liquid film. The powder covered stripis then subjected to a rolling operation to compact the powder. In thisstep, the powder is rolled into a flat, compacted metallic layer inwhich adjacent grains of powder are bonded together; This metallic layeris in a semi-adherent conditionin relation 7 to the strip, the undersideof the metallic layer having been mechanically bonded to the stripsurface. The composite article of strip and compacted metal powder issintered in a heat-treating furnace at a controlled temperature, and fora time period sufficient to form an adherent iron-nickel alloy on thesurface of the strip. Sintering is performed in a protective atmosphere,preferably one which is slightly reducing.

In the accompanying drawings:

FIG. 1 is a reproduction of a photomicrograph of a,

transverse section of a steel strip coated with an ironnickel alloywhich was sintered at 1725 F. for 3 hours.

FIG. 2 is a graph showing the effect of increased sintering time at 1725F. on surface nickel content.

FIG. 3 is a graph showing the effect of increased tem- FIG. 5 is areproduction of the line scan of X-ray intensity for the coating of FIG.1, as determined by electron microprobe.

FIG. 6 is a reproduction of a macrophotograph of a section of steelstrip coated with an iron-nickel alloy, which has been subjected to aSwift cup test and etched with acid.

Referring to FIG. 1 it will be seen, from the welldefined line at theinterface, that the alloy coating of our invention is a structuredistinct and separate from the iron base metal. The iron base stripmaterial used in the sample, from which the photomicrograph wasobtained, had a carbon content of 0.003%, a very ductile and desirabletype of iron where subsequent drawing operations are to be applied tothe coated article. However, it is not essential that the carbon be aslow as that shown for FIG. 1, Ductility requirements can be readily metwith carbon contents as high as 0.10%. Higher carbon contents in thesteel, those above about 0.10% to 0.15%, may lead to formation of porouscoatings. Generally, if excellent ductility of the coating is desired,satisfactory results can be obtained with carbon in the base stockranging up to 0.10%.

Our coating has several advantages over conventional nickel coating,including low production cost, and ease of producing the coating.Another advantage of the coating is its superior resistance to corrosionafter the coated product has been subjected to moderate deformation.

In one preferred method, by which the alloy coating of the invention maybe produced, a 20 gage strip of rimmed steel 5" wide, having a carboncontent of 0.06% is unwound from a coil and passed through a rollercoater bearing rubber applicator rolls, where a thin, uniform film oftridecyl alcohol is applied to both sides of the strip.

The strip is next passed through a fluidized bed of nickel powder. Thenickel powder used in the fluidized bed is of a size less than 200 mesh,and is made from carbonyl nickel, analyzing 99.9% Ni. The alcohol filmretains the powder layer on the strip in its passage to the nextsubsequent step, in which the strip is introduced into a set of 4 inchdiameter hard finished rolls in a horizontal plane. In this operation,the powder is compacted with the base metal strip at a pressuresufficient to produce a strip elongation of about 10%. After leaving therolls, the strip is wound on a take-up reel. The coil is unwound andrecoiled with a spacer-wire inserted between adjacent laps to produce anopen-coil effect. This spacing. of thelaps of the coil is necessary toprevent possible welding of adjacent laps in the subsequent sinteringtreatment, and to permit the sintering' gases to "circulate freely. Theopen-wound coil is placed in an annealing furnace, where it is heattreated at 1725 F. for 24 hours in a dry atmosphere of 18% hydrogen and82% nitrogen on a volume basis. After sintering, the strip is brightenedby cold rolling with polished rolls, to the extent of a 1% to 2%elongation of the strip.

In another example, exactly the same operating conditions are used asdescribed above, except that a 36 inch wide strip is used as the basestock, and compacting is performed by means of 26 inch diameter rolls.The wider, compacted strip has an elongation of from 2% to 5%.

' While the foregoing examples relate to strip, it should be noted thatthe alloy coating can be i'applied to other flatware such as sheets andbars, and to rounds such as rods and wire. The same ,majorsteps ofapplying the powder, compacting and sintering are. applicable to anytype of stock to which the coating'is applied, the method differing onlyin the particular manner of physically handling different types of basesteel. Whatever the stock used, it is preferable, during sintering, toprovide adequate spacing to prevent welding of adjacent parts of thearticle. When sheet or strip are coated, it is of course possible tocoat one side, or both, as desired.

Generally, if the strip is soiled, it should be cleaned with a cleaningmedium such as hydrocarbon solvent or an alkaline cleaner beforecoating.

The powder-retaining material, which is applied to the strip in the formof a thin film, and which acts temporarily as a powder-retaining medium,may be any liquid substance having the proper viscosity, volatility andtackiness characteristics, and which meets industrial safetyrequirements. The viscosity should, of course, be such that the film ismaintained on the base stock until after the powder has been compacted.The liquid should be completely volatile at the sintering temperature sothat no residue remains.

The metal powder can be applied to the steel backing member without aliquid substance having the above-stated characteristics first beingapplied, but the use of such film is preferred, for the film lendsmechanical efficiency to the powder applying and compacting steps.

Liquids, other than tridecyl alcohol, which may be used as effectivepowder retaining films, are kerosene, transformer oil, strarw oil andcertain naphthene base oils such as those having a Saybolt Universalviscosity of from 90 to 110 seconds at 100 F.

While not critical, it is desirable to control both the amount of liquidapplied and the grain size of the metal powder. The alcohol, or othersubstance, used for retaining the metal powder, should be applied in arather thin film of a thickness just sufiicient to cause adequateadherence of the powder particles. An excess of the liquid causesproblems of slippage and inefiicient compacting during the rollingoperation.

In respect to the nature of the powder which may be applied,electrolytic nickel powder, commerically pure nickel powder, nickeloxide, mixtures of nickel and nickel oxide powders, iron-nickel alloy,and mixtures of any of these with iron or iron oxide powder may be usedto form the iron-nickel alloy coating. In general, elemental metalpowders compact and sinter better than their oxides, or mixtures ofmetals and oxides. However, by psing a reducing sintering atmosphere,the nickel oxide can be completely reduced and the nickel alloyed withthe steel base.

The compositions of these powders, as defined in the appended claims,may include minor amounts of inert materials, or other elements which donot adversely affect the process.

Powders have been used having a particle size ranging from a mixture 50%of 140 mesh to +325 mesh with 50% of 325 mesh, down to 100% of 325 mesh.Mesh numbers given herein correspond to the United States Standard SieveSeries. Generally, the finer powders appear to permit a wider variationin compacting and sintering practice, including the amount of elongationof the base steel required during compacting, and/ or time andtemperature required in sintering. Coarser powders can be used to formthe nickel alloy coating, but such powders tend to form porous coatings.

Very satisfactory compacting and sintering results when the weight ofpowder applied is approximately 20 grams per square foot of steelbacking surface. Heavier or llghter applications of powder may be used,depending to some extent on the desired distribution of nickel in thealloy coating, and the desired thickness of the sintered coating.Satisfactory coatings have been made from powder in an amount of 8 gramsper square foot, and from powder in an amount of 40 grams per squarefoot. Coatings formed from applications of powder greater than 20 gramsper square foot will require correspondingly heavier applica tions ofpowder retaining fiuid.

Any relatively pure nickel powder, or nickel oxide powder, may be usedto develop the alloy coating. Trace impurities such as cobalt, copperand iron have no noticeable effect on the resultant coating. It shouldbe observed, however, that carbon content of the powder should be low,as earlier explained, if the coating is to be free of brittleness;satisfactory results have been obtained with powders containing up to0.17% carbon.

Examples of nickel powder used to produce satisfactory coatings include:Inco Grade A, Sherritt-Gordon F Grade, Metals Disintegrating Co. No. 151and Glidden Co. F110.

The powder is compacted to provide a uniform metal shell on the basemetal where individual powder particles are welded together, andparticles immediately adjacent the base metal are forced mechanicallyinto the base metal surface. The compacted shell of powdered metal isporous in nature, and this characteristic is advantageous in thesubsequent sintering step. During the heating-up period prior tosintering, the volatile oily material, originally applied to hold thepowder to the strip, is vaporized and escapes through the pores of thecompacted layer. If the compacted layer were not porous, the layer wouldbe lifted off by the vapors during sintering.

The roll pressure to be applied in the compacting step will vary withinwide limits, depending on the diameter of the rolls used and on powderparticle size. For example, with a rolling mill having 4-inch diameterrolls, the elongation of the base steel may vary from between 5% and50%, the finer powders yielding satisfactory coatings at allelongations, while with coarser powders, those having a particle size offrom 30% to 50% of +325 mesh, the elongation should be at least 10% toproduce a porefree coating after sintering. When rolls of 26 inchesdiameter were used in the compacting step, a base metal elongation ofonly 2% to 5% was satisfactory.

In the case of strip, the base metal may be either annealed cold rolledstrip, or pickled hot rolled strip. Any gage which can be open coilannealed is satisfactory.

The temperature during sintering should range, preferably, betweenapproximately 1550 F. and 1900 F. While 1400 F. is believed to be thelower limit at which satisfactory sintering can be performed, there is,in reality no upper limit, any practical working temperature above 1400F. being satisfactory. The minimum time period required for propersintering at 1400 F. is approximately 48 hours. For higher sinteringtemperatures, there is a decrease in the holding time requirement, untilat 2100 F., or higher, satisfactory coatings can be made with a holdingtime of 15 minutes. Sintering times may vary somewhat, depending on thethickness and nickel alloy content desired in the coating. While highsintering temperatures have no adverse effect on the resultant alloycoating, there are, of course, obvious objections to operating atexcessively high temperatures, temperatures above 2000 F.

Dry hydrogen, dry 18% hydrogen with 82% nitrogen, and dry 4% hydrogenwith 96% nitrogen atmospheres have been used as annealing (sintering)gas with equal success. For best results, the annealing atmosphereshould be. reducing to both nickel and the base steel; however, highpurity with regard to oxygen, water vapor or carbon contaminants is notnecessary for good alloy formation.

In lieu of the type of sintering atmosphere just described, a partialvacuum is quite satisfactory.

The choice of the type of base steel used is dependent on the propertiesdesired in the finished alloy-coated product. This, as has beenpreviously remarked, the carbon should be no higher than 0.1%, iffreedom from porosity is required. There is another consideration,involving possible loss of carbon due to the nature of the sinteringatmosphere. When producing our alloy coating on decarburized base stock,i.e., 0.01% or lower, there is no problem in retaining base stockproperties. If a higher carbon base stock, such as rimmed steel, isused, that is, 0.03% carbon or higher, there may be some loss of carbonin the base metal during sintering. Decarburization can occur duringsintering if the sintering atmosphere contains a high percentage ofhydrogen. Hence, when employing a base stock having, say, from 0.03% to0.05% carbon, where it is desired to retain strength properties and finegrain characteristics, the sintering atmosphere should be low inhydrogen and impurities which favor decarburization, such as water,oxygen and carbon dioxide. One example of a low hydrogen sinteringatmosphere already given is that having 18% hydrogen and 82% nitrogen.With sufficient sintering time and temperature, successful sintering maybe performed with a hydrogen-nitrogen atmosphere wherein the hydrogen isas low as 4%. e

X-ray analysis of the sintered coating indicated that the entire coatingconsists of an iron-nickel alloy. e

Referring to the drawings, FIGURE 1 represents a typical nickel alloycoating produced by our method. The sample was etched incyanide-persulfate and the photomicrograph was made at 250 diameters.The average thickness of this coating is 0.8 mil, as measured bymetallographic inspection from the outside surface to the interfacebetween alloy coating and metal substrate. Compacted samples sintered atother temperatures and times, produced coatings which varied inthickness from 0.5 to 2 mils. Depth of the coating is influenced by thetype of nickel powder used, Weight of powder applied, percent elongationin compacting, and sintering temperature and time.

The coating shown in FIGURE 1 possesses surface nickel equal to 59weight percent of the alloy coating. Other coatings varied from 28 to90% nickel at the outside surface of the coating. This surface nickelcan be varied by changing the sintering temperature and time. FIGURE 2shows the effect of variation in time at constant temperature. Increasedtime promotes diffusion between nickel and iron resulting in loweredsurface nickel content. FIGURE 3 shows the effect of variation intemperature with constant sintering time. Just as with increased time,increased temperature also favors diffusion, again lowering surfacenickel.

The curve in FIGURE 4 is developed from points representing actualnickel content at different positions on the line scan of FIGURE 5. Thescan was performed on the coating shown in the photomicrograph in FIG-URE 1. This coating was sintered for a relatively short time (3 hours),therefore nickel concentration is fairly high at the surface, droppingoff quite sharply as the coating-steel interface is approached. Highersintering temperatures and/or longer times act to lower surface nickel,while decreasing the steepness of the nickel graclient in the alloycoating.

High surface nickel content, produced by the lower temperature and/ orshorter time cycles is somewhat beneficial to corrosion resistance. Inone test, in which sintering was performed at 1400" F. for 48 hours, thenickel at the outside coating surface analyzed 90%. The longertime-higher temperature treatments, which lower surface nickel content,are effective in reducing voids to a minimum throughout the coating.This is helpful wherever coatings must undergo extensive deformation informing. Any voids within the coating could act as severe stressraisers, causing the coating to break apart.

To be truly pore-free and corrosion resistant to mineral acids, thenickel content near the surface should be at least 28%.

When sintering temperatures are about 1650 F., or above, the iron-nickelcoating alloy, and the base steel, will have an austenitic structure(face centered cubic) at the sintering temperature. Because of the highnickel content of the alloy, this structure will be retained through thecooling cycle, down to room temperature.

One of the outstanding characteristics of the alloy coating is therelative freedom from porosity. It is believed that the dense structureobtained is due, at least in part, to the method of alloy formation.During the heat-treating, or sintering, operation, nickel atoms diffuseinwardly to the base. At the same time, iron atoms diffuse outwardlyfrom the base into the compacted nickel powder. This diffusion firstcauses bonds, or bridges, of continuous 'metal between powder particles,and between powder par- 5 ticles and the base steel. This actionproduces voids between particles. As ditfusion progresses, it reducesthe surface area of the voids to a point where the voids are 'verysmall, or do not exist.

Another characteristic of the coating is its ductility. 1O Coated sheetproducts are able to withstand bending of 180 over a radius of twice thesheet thickness, without harming the integrity of the coating.

Corrosion resistance of the alloy coating is excellent, as indicated byaccelerated tests. For example, specimens of our alloy coated product,which had been rolled to a bright finish and which were immersed in tapwater for a period of six months at room temperature, exhibited no signsof rusting or discoloration.

As another example of corrosion resistance, a specimen 4 inches by 4inches of our alloy coated product was immersed in 6.; weight percentsulfuric acid at 180 F. for one hour. Similiar tests were made on thebase stock and on the base stock electroplated with one mil of nickel.The weight loss results in grams/sq. ft. of surface area/ hour, areshown in the table below:

Weight loss in A further example of the corrosion resistance of thismaterial is shown by performance of a standard salt fog test, made inaccordance with A.S.T.M. Specification B117-61. Four inch by six inchspecimens, buffed to a highly polished surface finish, showed no signsof corrosion after 24 hours.

Furthermore, a coated article made by our process can be reduced incross-section by as much as 80% and still withstand immersion in tapwater for 60 days without display of any porosity.

As an example of the ductility of coatings made by our method, a 20 gagestrip of rimmed steel, having a carbon content of 0.06% was filmed withalcohol, coated in a fluidized bed with nickel powder of a particlesize-Jess than 325,mesh, andthe powder compacted "on the'strip at apressure equivalent to a strip elongation of about 10%. Aftercompacting, the strip was recoiled with spacer wire to produce an opencoil, and annealed in an annealing furnace at 1725 F. for 24 hours in adry atmosphere of 18% hydrogen and 82% nitrogen. After" sintering,'the

strip was cold rolled with polished rolls to a reduction incross-sectional area of 50%. The rolled strip was then continuouslyannealed at 1250 F. in an atmosphere of 4% hydrogen and 96% nitrogen.

The annealed coated strip had a thickness of 0.018 inch,

or a gage of approximately 26. Coated strip of this gage can be madeonly by reduction of cross-sectional area after the sintering step, asthe sintering step is limited to about 24 gage strip. Strip below thisgage would collapse in the sintering furnace.

After reduction of the coated strip, in the range of from 5 to 80percent, the ductility will be reduced to the point where use of theproduct will be limited to those applications requiring a minimumbfdeformation. If additional formability is required for the intendedapplication, the product should be annealed after cold rolling torestore its ductility.

The coating of the cold rolled, annealed product of the previous examplewas continuous and pore-free.

A pore-free coating, as the term is used herein, refers to that type ofcoating which has no discontinuities, or

holes, extending from the coating surface to the base steel,

and is not meant to exclude small openings in the body of the coating,which do not penetrate the entire coating thickness.

Both the ductility characteristic, and the corrosion resistance of theproduct made by our invention, is shown graphically in FIG. 6.

The macrophotograph, made at 2 magnifications, represents the cupresulting from a Swift cup test. A round test specimen, 4 inches indiameter was prepared from a decarburized rimmed steel base metal having0.003% carbon. The alloy coating, containing a minimum of 30% nickel atthe coating surface, has a thickness of approxi mately 1 ml. The samplewas formed into cup by the conventional Swift cup test procedure, andthe formed cup was immersed in 6.5 weight percent sulfuric acid for 8hours at 180 F., suflicient time to permit the steel substrate to beremoved to a depth of 1.5 millimeters at the edge of the cup. Theductile iron-nickel alloy coating shows no degradation from the acidtreatment.

The method of performing the Swift cup test is described in an articleby O. H. Kemmis in Sheet Metal Industries at vol. 34, 1957, pages 203and 251.

Another example of coating ductility is shown in the ability of ourcoating to undergo deformation by the Olsen cup test, without anydecrease in its ability to protect the base steel from corrosion.Samples deformed to a cup height of 0.30 inch, and immersed in tap waterat room temperature, showed no rusting after 90 days exposure. Bycontrast, samples of electroplated nickel coattings of comparablethickness (about 1 mil) showed loss of corrosion resistance withdeformations as low as 0.10 in. in Olsen cup height after one hourexposure in tap water.

The procedure for the Olsen test is described in Making, Shaping andTreating of Steel, 7th Edition, 1957, pages 923-924.

The dense structure of our alloy, free from pinholes,

makes this alloy admirably suitable as an undercoat for a chromiumelectroplated finish. Because of the porosity of chromium plate, it isnecessary to form an undercoat of a metal such as nickel before thechromium is applied. Our alloy can perform the function of a chromiumundercoat in the same manner as electrodeposited nickel, and 'withequally satisfactory results.

An added advantage of an alloy coating, as an undercoat for an articlehaving an outer surface of electrodeposited chromium, is that thearticle can be deformed after electroplating without loss of corrosionresistance. In a tap water test, samples of our nickel alloy coating,superimposed with an electrodeposited chromium layer of a thickness of10 millionths of an inch, were deformed to an Olsen cup height of 0.3inch, and immersed in the water for 24 hours. At the end of theimmersion period,

the deformed samples showed no loss of corrosion resistance. Bycontrast, a sample having a similar coating of chromium, over a 1 milthick electrodeposited nickel coating, and deformed to a 0.1 inch Olsencup height, failed after a 1 hour immersion in tap water.

The nickel alloy coating may be buffed to a bright finish, when a shiny,pleasing appearance is desired. The buffed coating is especiallyadvantageous when the nickel alloy coating is used as an undercoat forchromium.

In addition to chromium, our alloy coating may be electroplated withother metals, for example, nickel.

We claim:

1. An article comprising a ferrous base having a ductile, continuous,pore-free coating, said coating consisting of an austenitic alloy ofiron and nickel wherein the nickel content at the surface ranges frombetween and 28 weight percent of the alloy.

2. An article comprising a ferrous base, a continuous, pore-free coatingon said base, said coating consisting of an austenitic alloy of iron andnickel, and a metallic chromium coating completely enveloping said alloycoating.

3. An article comprising a ferrous base and a continuous, pore-freecoating on said base, said coating comprising a ductile austenitic alloyof iron and nickel.

4. An article comprising a ferrous base containing not more than 0.1%carbon and having a continuous, porefree coating, said coatingcomprising an austenitic alloy of iron and nickel wherein the nickelcontent at the surface ranges between 28 and 90 weight percent of thealloy.

5. An article comprising a ferrous base, a continuous, pore-free coatingon said base, said coating consisting of an austenitic alloy of iron andnickel, and a metallic coating of a member of the group consisting ofchromium and nickel completely enveloping said alloy coating.

6. An article comprising a ferrous base having a ductile, continuous,pore-free coating of an austenitic alloy of iron and nickel, saidcoating containing iron diffused from said base and having a nickelcontent at the surface ranging from 90 to 28 weight percent of the alloyand a decreasing nickel concentration gradient to the coating interface.

References Cited UNITED STATES PATENTS 2,292,694 8/1942 Jerabek 29182.3X 2,350,179 5/1944 Marvin 29-182.3 X 2,872,311 2/1959 Marshall et al.29-182 3,094,415 1/1963 Gallatin 29-1823 X 3,166,796 l/l965 Wehinger29-196.1 X

HYLAND BIZOT, Primary Examiner.

4. AN ARTICLE COMPRISING A FERROUS BASE CONTAINING NOT MORE THAN 0.1%CARBON AND HAVING A CONTINUOUS, POREFREE COATING, SAID COATINGCOMPRISING AN AUSTENITIC ALLOY OF IRON AND NICKEL WHEREIN THE NICKELCONTENT AT THE SURFACE RANGES BETWEEN 28 AND 90 WEIGHT PRECENT OF THEALLOY.